Resistance variable memory elements, which include electrokinetic memory elements using chalcogenides, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A typical chalcogenide resistance variable memory element is disclosed in U.S. Pat. No. 6,348,365 to Moore and Gilton.
In a typical chalcogenide resistance variable memory element, a conductive material, for example, silver, tin and copper, is incorporated into a chalcogenide glass. The resistance of the chalcogenide glass can be programmed to stable higher resistance and lower resistance states. An unprogrammed chalcogenide variable resistance element is normally in a higher resistance state. A write operation programs the element to a lower resistance state by applying a voltage potential across the chalcogenide glass and forming a conductive pathway. The element may then be read by applying a voltage pulse of a lesser magnitude than required to program it; the resistance across the memory device is then sensed as higher or lower to define two logic states.
The programmed lower resistance state of a chalcogenide variable resistance element can remain intact for an extended period of time, typically ranging from hours to weeks, after the voltage potentials are removed; however, some refreshing may be useful. The element can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the device to the lower resistance state. Again, the higher resistance state is maintained in a semi- or non-volatile manner once the voltage potential is removed. In this way, such an element can function as a semi- or non-volatile variable resistance memory having at least two resistance states, which can define two respective logic states, i.e., at least a bit of data.
One exemplary chalcogenide resistance variable device uses a germanium selenide (i.e., GexSe100−x) chalcogenide glass as a backbone. The germanium selenide glass has, in the prior art, incorporated silver (Ag) and silver selenide (Ag2+/−xSe) layers between first and second electrodes in the memory element. Such elements are programmed by applying a sufficient voltage across electrodes to cause the formation of a conductive path between the two electrodes by virtue of a conductor (i.e., such as silver) that is present in metal ion laced glass backbone.
Although the silver-chalcogenide materials are suitable for assisting in the formation of a conductive channel through the chalcogenide glass layer for silver ions to move into, other non-silver-based chalcogenide materials may be desirable because of certain disadvantages associated with silver use. For example, use of silver-containing compounds/alloys such as Ag2Se may lead to agglomeration problems in the electrokinetic memory device layering and Ag-chalcogenide-based devices cannot withstand higher processing temperatures, e.g., approaching 260° C. and higher. Tin (Sn) has a reduced thermal mobility in GexSe100−x compared to silver and the tin-chalcogenides are less toxic than the silver-chalcogenides.
Research has been conducted into the use of thin films of SnSe (tin selenide) as switching devices under the application of a voltage potential across the film. It has been found that a 580 Å SnSe film shows non-volatile switching between a higher resistance state (measurable in MOhm) and a lower resistance state (measurable in kOhm) when potentials of 5-15 V are applied by forming an Sn-rich material (e.g., a dendrite). Also, the addition of Sn to a GexSe100−x glass, which is a chalcogenide glass, has been found to produce memory switching if a high enough potential, e.g., >40 V, is applied across the chalcogenide glass. However, such switching potentials are too high for a viable memory device.